Do you know if you’ll go off the beaten path at all?  Will you go hiking through hills, or along Brittany’s rocky coast?

Or will you only be spending time in cities and towns, walking on pavement the whole time? 

If the answer is yes to the first question, a good pair of comfortable hiking boots may be needed.  If the answer is yes to the second question, then a pair of sneakers will do. 

However, if you’re going in May and will be doing a lot of walking, there may also be some hot days, especially in France.  Have you considred hiking sandals?

I’m no footwear expert by any means, but just use the same guidelines you’d be using at home. What I would advise against, however, is bringing multiple pair of shoes/boots. They just weigh you down; even if you’re going to be doing a mix of cities and countryside/outdoorsy places, just pick one type of shoe or the other. The worst possible thing you could do is spend 90% of your time in Stockholm with one sidetrip out to Dalarna countryside for instance, and be lugging around your shiny new Timberlands on your back most of your trip
 
On the other hand, I’d advise against sandals too, unless you are used to wearing them every single day at home.

There are plenty of lightweight hiking boots out there just go and have a look in any decent outdoor store, I agree with the above comment on hiking sandals, unless you are used to wearing them breaking in a pair on a trip would not be fun, plus a sandal can never replace a good boot.

I have to have two pairs of footwear – whether that’s one pair of walking shoes and one pair of sandals, or one pair of hiking boots and one pair of walking shoes, or whatever. If I don’t switch footwear now and then, my feet get very unhappy.